1. Field of the Invention
This invention relates to a method of treating an aqueous liquid containing halogenated organic impurities. More particularly, this invention relates to a method of treating an aqueous liquid containing halogenated organics using a combination of light energy and ultrasonic energy in the presence of a photocatalyst to decompose the halogenated organic impurities in the liquid.
2. Description of the Related Art
It is desirable to remove halogenated organic materials from aqueous liquids such as water containing chlorinated hydrocarbons, e.g., chlorinated phenols. Prior art removal techniques have included the use of ultraviolet light radiation to decompose halogenated organic compounds. For example, Chou et al. U.S. Pat. No. 4,764,278 discloses a method for reducing the concentration of haloorganic compounds in water by first extracting the haloorganic compounds from the water using a water immiscible alkane hydrocarbon solvent. The solvent is then separated from the water and regenerated by exposing the solvent to ultraviolet light to degrade the haloorganic compounds.
Ultraviolet light energy has also been used in combination with a photocatalyst, sometimes also referred to as a semiconductor photocatalyst, such as TiO.sub.2, to remove halogenated organic materials from aqueous liquids by decomposure of the organic contaminant. For example, Barbeni et al., in "Photodegradation of 4-Chlorophenol Catalyzed by Titanium Dioxide Particles", Nouveau Journal de Chimie, Vol. 8, (1984), pp. 547-550, describes the decomposition of 4-chlorophenol in an aqueous solution containing a suspension of TiO.sub.2 exposed to radiation of UV wavelength or sunlight to form CO.sub.2 and HCl.
D'Oliveira et al., in "Photodegradation of 2- and 3-Chlorophenol in TiO.sub.2 Aqueous Suspensions", Environ. Sci. Technol., Vol. 24, No. 7, (1990), pp. 990-996, discuss the use of TiO.sub.2 in combination with radiation of &gt;290 nm., and preferably &gt;340 nm., to remove 2-chlorophenol and 3-chlorophenol.
Ollis, in "Contaminant Degradation in Water", Environ. Sci. Technol., Vol. 19, No. 6, (1985), pp. 480-484, discloses the removal of trichloromethane (chloroform) or ethylene dibromide from an aqueous solution by simultaneous presence of both TiO.sub.2 and near-UV light of 300 nm. to &lt;400 nm. In both cases, use of either the catalyst or the UV illumination alone did not produce the degradation.
Zepp, in "Factors Affecting the Photochemical Treatment of Hazardous Waste", Environ Sci. Technol., Vol. 22, No. 3, (1988), pp. 256-257, discusses various photoreactions, pointing out that direct photoreactions of ionizable compounds, such as chlorophenols, are often very sensitive to pH. He also observes that the combination of UV light and ozone is effective for oxidizing pollutants. He states that irradiated semiconductors are versatile reagents that show promise for treatment of hazardous wastes and that titanium dioxide has been shown to effectively photocatalyze the reduction of chlorinated organics.
Ultrasonic energy has also been used in the removal of halogenated organics from an aqueous liquid. For example, Sittenfield U.S. Pat. No. 4,477,357 describes a process for removal of contaminants such as halogenated organics from a liquid. Halogenated organic materials in oil or water are mixed with an equal amount of an alkaline agent, such as a hydroxide or a carbonate of an alkali metal or an alkaline earth metal, and then exposed to ultrasonic energy to decompose the halogenated organic contaminant. The presence of the alkaline agent is said to significantly accelerate the dehalogenation and decomposition of organic ring structures.
Sierka et al., in "Catalytic Effects Of Ultraviolet Light And/Or Ultrasound On The Ozone Oxidation Of Humic Acid and Trihalomethane Precursors", describe the catalytic effects of the use of both UV irradiation and ultrasound, either singly or in combination, on the ozone oxidation of organic materials, such as humic acid, in aqueous solutions. It is indicated that the most effective reactor conditions for both the destruction of nonvolatile total organic carbon and trihalomethane formation potential utilized both ultrasound and UV irradiation in combination with ozone.
While these methods have been shown to be successful in removing halogenated organic contaminants from aqueous liquids and decomposing such organic materials, usually the reaction times are sufficiently slow to reduce the economic attractiveness of such processes, especially for continuous or on line treatment systems.